The present invention relates to the field of solar cells and methods for forming solar cells from semiconductor materials.
In order to utilize solar energy to generate electricity, solar cells made of monocrystalline semiconductor wafers or discs can be used only for special applications. Their manufacturing cost and the amount of energy consumed in their manufacture is so high that the use of monocrystalline cells makes sense only together with solar radiation concentrators. With such concentrators, care must be taken that sufficient cooling is provided for the solar cells.
Much more economical solar cells can be manufactured from amorphous or polycrystalline semiconductor layers. Amorphous silicon layers, for example, can be produced by vapor deposition of silicon in a vacuum or by cathode atomization or high frequency atomization of silicon in a hydrogen containing atmosphere. While solar cells of amorphous silicon have the drawback that their degree of efficiency is only a few percent, solar cells of polycrystalline silicon can be produced with degrees of efficiency 10%. Particularly high efficiency is produced by a polycrystalline silicon material in which the crystallites have a preferred direction. The polycrystalline silicon for solar cell production is manufactured, for example, by casting liquid silicon in the form of a bar having a square cross section. The liquid silicon solidifies into bars with a temperature gradient. After cooling, the solidified bar is sawed or sliced into wafers from which the solar cells are made. Alternatively, the liquid silicon can be cast directly in the form of a plate, eliminating the requirement for sawing or slicing.
It is also known to immerse a suitable substrate, e.g., graphite, into a silicon melt so that the substrate when removed from the melt, is covered with a thin coating of silicon. This silicon coating on the substrate can then be processed further into a solar cell. A substrate with a coating of a Group III/Group V compound, such as a thin coating of GaAs, can be produced in a similar manner and it can then be processed further into a solar cell.
It is known to form a polycrystalline semiconductor layer solar cell by employing a plasma spray to form the semiconductor layer as disclosed in U.S. Pat. No. 4,003,770 to Janowiecki et al.